1. Technical Field
The present invention relates to a subscriber's telephone circuit equipped with a repertory memory powered from a self-contained battery, and more particularly to circuitry to protect such a battery while maintaining sufficient voltage to maintain the information stored in the memory.
2. Background Art
With the introduction of electronic telephones, it has become not only feasible but common to provide repertory memory dialing functions with the ability to store, recall and automatically dial out telephone numbers of fifteen or more digits. In such an arrangement, usually ten or more memory registers, each capable of storing a complete sequence are provided using integrated circuits, such as the MOSTEK MK-50981 and MK-5175 integrated circuit units, that contain the necessary memory registers and control functions. The memories contained in such integrated circuits are usually of of the volatile type and require constant external current to maintain information stored in the memory. A number of methods are employed for such maintenance of information in the memory during such time as the telephone is in the "on-hook" condition. Such methods include the utilization of a long life non-customer replaceable battery, the use of a customer replaceable battery, connection to an external power supply from a 110 volt AC line, or the powering of the memory unit from the telephone line by means of utilization of a high value resistor (10 megohms or greater) to drain a few microamps from the telephone line. Only the use of batteries have the potential for maintaining the memory continuously without interruption by telephone line voltage transients or power outages.
Even with the utilization of a long life noncustomer replaceable battery, subscribers have often become dissatisfied due to an unexplainable loss of memory of a temporary nature, thus requiring restoring all of the information lost. Usually when such a memory loss occurs, all of the stored numbers are lost and must be reentered and thus restored to return the telephone to the repertory mode of operation. The major cause of such memory loss is due to transients in the memory power supply circuit which caused the memory retention current to drop below the necessary threshhold to maintain retention. Such transients can be produced by the telephone dial pulses from a key controlled pulse generator which is not adequately decoupled from the memory circuit. It is also possible for external transients on the telephone line to cause the memory retention current to drop below the aforementioned threshhold.
Where the memory circuitry is powered from the telephone line directly, employing a high value resistor, only a capacitor is used to store a charge on the pulse generator circuitry. This simple arrangement is quite satisfactory for dial pulse only circuits, but is completely unsatisfactory for some dual tone multifrequency circuits due to the shunting effect on dual tone multifrequency toning levels. Accordingly, it is the object of the present invention to provide a new and useful protective circuit for repertory memories as employed in telephone subscriber circuits, which function with both dial pulse and dual tone multifrequency integrated circuits and with so-called combination dial pulse and dual tone multi-frequency circuits.